[Macros] Add Macro.inferNonisolatedConformances to control inference of nonisolated conformances

When this property returns true, infer nonisolated conformances for the
produced macro expansion. Do this by default, so that existing macros
will work with isolated conformance inference. Macros can opt out by
returning false, in which case we will skip this transformation.

Author: DougGregor

Sources/SwiftSyntaxMacroExpansion/MacroExpansion.swift

@@ -520,7 +520,12 @@ fileprivate extension SyntaxProtocol {
     for macro: Macro.Type,
     indentationWidth: Trivia?
   ) -> String {
-    let syntax = Syntax(self)
+    var syntax = Syntax(self)
+
+    // Infer nonisolated conformances.
+    if macro.inferNonisolatedConformances {
+      syntax = syntax.inferNonisolatedConformances()
+    }
 
     // Formatting.
     switch macro.formatMode {

Sources/SwiftSyntaxMacros/MacroProtocols/Macro.swift

@@ -15,4 +15,29 @@ public protocol Macro {
   /// How the resulting expansion should be formatted, `.auto` by default.
   /// Use `.disabled` for the expansion to be used as is.
   static var formatMode: FormatMode { get }
+
+  /// Whether to infer "nonisolated" on protocol conformances introduced in
+  /// the macro expansion when there are some nonisolated members in the
+  /// corresponding declaration group. When true, macro expansion will adjust
+  /// expanded code such as
+  ///
+  ///     extension C: P {
+  ///       nonisolated func f() { }
+  ///     }
+  ///
+  /// to
+  ///
+  ///     extension C: nonisolated P {
+  ///       nonisolated func f() { }
+  ///     }
+  ///
+  /// This operation defaults to `true`. Macros can implement it to return
+  /// `false` to prevent this adjustment to the macro-expanded code.
+  static var inferNonisolatedConformances: Bool { get }
+}
+
+extension Macro {
+  /// Default implementation of the Macro protocol's
+  /// `inferNonisolatedConformances` that returns `true`.
+  public static var inferNonisolatedConformances: Bool { true }
 }

Tests/SwiftSyntaxMacroExpansionTest/ExtensionMacroTests.swift

@@ -251,6 +251,41 @@ final class ExtensionMacroTests: XCTestCase {
       indentationWidth: indentationWidth
     )
   }
+
+  func testNonisolatedConformances() {
+    struct NonisolatedConformanceMacro: ExtensionMacro {
+      static func expansion(
+        of node: AttributeSyntax,
+        attachedTo declaration: some DeclGroupSyntax,
+        providingExtensionsOf type: some TypeSyntaxProtocol,
+        conformingTo protocols: [TypeSyntax],
+        in context: some MacroExpansionContext
+      ) throws -> [ExtensionDeclSyntax] {
+        return [
+          ("""
+          extension \(type): P {
+            nonisolated func f() { }
+          }
+          """ as DeclSyntax).cast(ExtensionDeclSyntax.self)
+        ]
+      }
+    }
+
+    assertMacroExpansion(
+      "@NonisolatedConformance struct Foo {}",
+      expandedSource: """
+        struct Foo {}
+
+        extension Foo: nonisolated P {
+          nonisolated func f() {
+          }
+        }
+        """,
+      macros: [
+        "NonisolatedConformance": NonisolatedConformanceMacro.self
+      ]
+    )
+  }
 }
 
 fileprivate struct SendableExtensionMacro: ExtensionMacro {